<P CLASS=INFO>The idea behind the MBHP_AOUT_LC module was to realize control outputs with parts that are easy to get in most countries. I started with this project after a discussion with Karl Dalen about the difficulity to find DIY friendly DACs with a serial interface. He proposed <A HREF="http://www.midibox.org/users/kd/KDdac.pdf">these circuits</A>, and I selected the discrete solution because of the simplicity. Some people requested to bring a ready made PCB into the MIDIbox Hardware Platform, so here it is!</P>

<P CLASS=DESC>In difference to the <A HREF="mbhp_aout.html">MBHP_AOUT</A> or <A HREF="">MBHP_AOUT_NG</A> module, this project doesn't require a special (expensive and hard to get) DAC IC, instead resistors are formed to a discrete R-2R ladder network. Building such a circuit means a lot of soldering effort, but as the MBHP is indented as a Do-It-Yourself platform, and not as a commercial product, this argument is very weak. However, the resistor ladder could be replaced by a DAC with parallel interface (see Karl's <A HREF="http://www.midibox.org/users/kd/KDdac.pdf">schematics</A>), this would still have the advantage, that the DACs are accessible via a serial chain (-> low number of uC pins are allocated), and that the software driver doesn't need to be adapted for a proprietary interface.</P>

<P CLASS=DESC>Regarding the quality of the "discrete DAC" outputs it should be mentioned, that it directly depends on the chosen resistors. The tolerance should be 1% or lower (Metalfilm) and stable over the typical temperature range. The results can be dramatically improved by selecting the best matching resistors: just buy a set of 100 pieces (e.g. at <A HREF="http://www.conrad.com" TARGET="_blank">Conrad</A>: a set of 100 * 10k metalfilm resistors for 1.33 EUR, part number #408280), measure the resistance and take the ones with the best matching values for the ladder circuit.</P>

<P CLASS=DESC>Another typical problem of cheap DACs (not only of this circuit, but also for cheap integrated DACs) is, that there is some "zipper noise" each time a new voltage is selected. The intensity of this noise depends on the bits which are toggled when a new voltage is selected - the leftmost bits (most signifigant bits) will produce the highest zipper noise. Even I can see this noise on my scope, I haven't noticed an audible effect with a discrete Moog- and CEM3378 filter, yet. Also CV controlled Korg MS-20 oscillators are working well with it. However, this is an explicit warning for those who expect perfect results! The zipper noise can be completely eliminated by adding a S&H circuit behind the DAC. By using the <A HREF="mbhp_shx8.html">NJU7304</A> this would also increase the number of available channels - on the other hand this solution requires once again a special IC or a lot of additional discrete ciruitry - it's a disaster! :-/</P>

<P CLASS=DESC>For those who don't feel disappointed yet, here two audio examples which demonstrate which results can be expected when the AOUT_LC module is connected between a MIDIbox SID and a CEM3378 filter: <A HREF="http://midibox.org/forums/topic/5158-cem3378-filter-first-impressions/" TARGET="_blank">Forum Posting #1</A>, <A HREF="http://midibox.org/forums/topic/8846-cem3378-schematic/?p=67098" TARGET="_blank">Forum Posting #2</A>.</P>

<TD COLSPAN=4 CLASS=TABCOLOR1>PCB data, can be viewed, modified and converted with <A HREF="http://www.cadsoft.de/download.htm" TARGET="_blank">Eagle Light</A>. The .pdf based schematic has been created with <A HREF="http://opencircuitdesign.com/xcircuit" TARGET="_blank">xcircuit</A>. There are no special eagle schematics available, since components have been netlisted in the .brd file directly!</TD>

<P CLASS=DESC>In order to increase the flexibility, the MBHP_AOUT_LC PCB provides two different resolution options. The first channel can either be controlled with 12bit or 8bit resolution, the second channel with 4bit or 8bit resolution:</P>

In the <A HREF="midibox_cv_v1.html">MIDIbox CV V1</A> firmware the resolution option is selectable. 8/8bit is enough if the module should control V/Oct CV inputs via Note or CC events. 12/4bit should be selected if channel #1 is connected to a Hz/V input, or if high-resolution events (NRPNs, PitchWheel) are desired.</P>

<TD CLASS=TABCOLOR2>Start with soldering the 7 bridges which save you from creating a 2-layer board. You can use the cutted legs of caps and resistors, before you throw them away after you finished all the modules! :-)</TD>

<P CLASS=DESC><A HREF="midibox_sid.html">MIDIbox SID V1</A> users: open the "setup_*.asm" file of the application and set the "ENABLE_AOUT" switch to 2. Thereafter <A HREF="howto_tools_gpasm.html">rebuild</A> the project and upload the new .hex file to the core. Now you can control channel #1 with the filter knob, and channel #2 with the resonance knob. If you haven't built the control surface, then just send CC#46 (CutOff) and CC#47 (Resonance) to the MIDIbox.<BR>The fun begins once the filter output voltage is modulated by the envelopes and LFOs! :-)</P>

<P CLASS=DESC><A HREF="midibox_fm.html">MIDIbox FM</A> users: open the "setup_*.asm" file of the application and set the "ENABLE_AOUT" switch to 2. Thereafter <A HREF="howto_tools_gpasm.html">rebuild</A> the project and upload the new .hex file to the core. AOUT channel #1/#2 is assigned to Instrument #1, Channel #3/#4 to Instrument #2, Channel #5/#6 to Instrument #3, Channel #7/#8 to Instrument #4. LFO/Envelope Assignments and the Offset can be configured in the AOUT menu of the Control Surface.</P>

<P CLASS=DESC><A HREF="midibox_sid.html">MIDIbox SID V2</A> users: open the "setup_*.asm" file of the application and set the "ENABLE_AOUT" switch to 2. Thereafter <A HREF="howto_tools_gpasm.html">rebuild</A> the project and upload the new .hex file to the core. You can control channel #1 with the filter knob, and channel #2 with the resonance knob with F2A option enabled (-> Ensemble Menu). With two chained MBHP_AOUT_LC modules, you can control CutOff and Resonance of the right channel SID the same way. With V2A, volume can be handled a similar way (4 chained AOUT_LC modules required for this case). With F2A and V2A option disabled, up to 8 channels can be controlled from the modulation matrix and within the EXT page. All these possibilities are documented in the <A HREF="midibox_sid_manual.html">MIDIbox SID Users Manual</A>.</P>